Prior Art
Railroad grade crossing for one-lane or two-lane highways are customarily marked by warning flasher signal lights and a cross-buck "Railroad Crossing" sign, both mounted on a roadside post or mast. Servicing of such roadside signal lamps may be performed conveniently at the roadside, causing no interference with highway traffic.
Grade crossings for wider, multiple-lane highways require grade crossing signal lamps suspended over the highway approach lanes for maximum visibility. For stability, these suspended warning flasher lamps are customarily mounted permanently on cantilever arms projecting from roadside masts, minimizing wind-tossing of the signal lamps, and providing maximum directional stability for the installed flasher signal lamps.
Maintenance, repair or replacement of such over-the-highway, suspended flasher signal lamps require convenient access without interfering with normal traffic flow. This may be achieved by mounting the signal-supporting cantilever arm on the upper portion of a segmented two-part mast, with the parts joined together by a rotational bearing assembly, as in my U.S. Pat. No. 3,444,512, thus allowing the cantilever arm to be swung aside to a position beside the roadway where it may be reached from a cherry picker crane or an access ladder, permitting maintenance and lamp replacement without obstructing highway traffic in any way. U.S. Pat. No. 3,321,160 shows another rotating-mast cantilever arm signal device.
Non-rotatable cantilever arms can be serviced from temporary structures, cherry-picker vehicles or ladders standing in the roadway, but these block traffic and create the risk of accidents, To avoid these disadvantages, a catwalk may be mounted on the cantilever arm, providing access for maintenance personnel up the mast and across the catwalk to reach the lamps mounted at the remote end of the cantilever arm.
To minimize torsional vibratory effects caused by air circulation and oscillating multiple vortex formation under high wind conditions, risking torsional twisting and vibratory or fatigue failure of such cantilever signal mast structures, the signal lamp units presenting a comparatively large projected area or "windage" facing the wind direction are preferably mounted with their center of effort coinciding with the principal axis of the cantilever arm, thus tending to balance and streamline the structure and reduce such vibratory forces. Lamps positioned at this level are difficult to reach, being at or below the level of the catwalk, and requiring gymnastic contortions by the repair man reaching overboard to service the relatively heavy lamp structures.